1. Field of the Invention
The present invention is related in general to bleaching of cellulose pulps. More specifically, the present invention concerns methods of bleaching cellulose pulps with chlorine dioxide.
2. Related Art
Bleaching of cellulose pulps typically employs chlorine containing chemicals. Efforts have been made to reduce their use because of pollution concerns, however their use has continued. This due at least in part because the brightness of the paper made from the pulps obtained using other oxidizers is either obtained at greater cost, and/or the strength of the paper ultimately produced from the bleached cellulose pulp does not meet manufacturer and consumer requirements.
Some pulps processors using chlorine dioxide will use the chlorine dioxide to bleach cellulose pulp in the initial pulp bleaching stage or stages, and then, in additional stages, add a non-chlorinated oxidizer for meeting final brightness specifications of the paper. This approach, while commendable because the final strength of the paper is good, leads to only a small reduction in actual chlorine dioxide usage. Other processors have added chlorine dioxide and non-chlorine-containing oxidizers to pulp to be bleached in one and the same stage; however, these pulp processors have not adjusted the ratio of chlorine dioxide to non-chlorine-containing oxidizer in a manner that affects strength of the final paper produced. Indeed such a relationship of this ratio to paper strength has heretofore not been recognized by prior practitioners. The only direction given on how to achieve good strength characteristics of the final paper when using ozone and chlorine dioxide is the viscosity of the treated pulp. However, a drawback of the viscosity test as a selectivity rating is that it is known that the correlation between pulp viscosity and strength characteristics of ozone-treated pulps differs from that of chlorine-treated pulps. (It can also be seen in Example-2/Table 2 & FIGS. 3 & 4) Depending on the reaction conditions and the treatment history of the pulp, ozone-treated pulps can have much lower viscosity values than pulps bleached with chlorine-based sequences to the same strength (see Pulp Bleaching: Principles and Practice, by C. Dence and D. Reeve, p. 734, published 1996 by TAPPI Press).
There is thus a genuine need in the art of cellulose pulp bleaching for methods of bleaching these pulps using chlorine dioxide and ozone in ratios that do not sacrifice pulp brightness or strength characteristics of the final paper made from the bleached pulps, while also keeping in mind the strict environmental rules and regulations now in force in many regions of the world.